Electroencephalograph monitoring apparatus

ABSTRACT

1. APPARATUS FOR MONITORING ELECTRIC SIGNALS IN THE HUMAN BRAIN, COMPRISING: A PAIR OF ELECTRODES, ADAPTED TO BE FIXED TO THE SCALP, FILTER MEANS COUPLED TO SAID ELECTRODES, HAVING A FREQUENCY VERSUS LOSS CHARACTERISTIC IN WHICH LOSS DECREASES WITH INCREASING FREQUENCY SO AS TO, IN OPERATION, SUBSTANTIALLY EQUALIZE THE AMPLITURE OF THE FILTER OUTPUT SIGNALS DUE TO RANDOM ELECTRICAL ACTIVITY OF THE BRAIN, THE SAID DECREASE IN LOSS WITH INCREASE IN FREQUENCY BEING FROM A LOW FREQUENCY DESIGNATED THE LOWER PREDETERMINED FREQUENCY, TO A HIGH FREQUENCY DESIGNATED THE UPPER PREDETERMINED FREQUENCY, THE UPPER AND LOWER PREDETERMINED FREQUENCIES BEING BELOW 30 C/S AND ABOVE 0.3 C/S, RESPECTIVELY, AND LOSS BELOW THE LOWER PREDETERMINED FREQUENCY, AND ABOVE THE UPPER PREDETERMINED FREQUENCY, BEING GREATER THAN THE LOSS AT THE LOWER AND UPPER PREDETERMINED FREQUENCIES, RESPECTIVELY, AMPLIFYING MEANS FOR AMPLIFYING SIGNALS WHICH PASS THROUGHH SAID FILTER MEANS, AND INDICATOR MEANS FOR INDICATING THE MAGNITUDE OF SIGNALS FROM SAID AMPLIFYING MEANS.

Oct. 29, 1974 MAYNARD Re. 28,214

BLECTROENCEPHALOGRAPH MONITORING APPARATUS Original Filed July 13. 19716 Sheets-Sheet I LOG V A 1974 D. E. MAYNARD ELECTRQENCEPHALOGRAPHMONITORING APPARATUS 6 Sheets-Sheet 5 Original Filed July 13. 1971 NEEE24 925.3403 oh ESE 5 E In? mmSZ 33 V QM IPUU NGQEB II II III MERE mm- Lfifimdmm E Oct. 29, 1974 D. E. MAYNARD Re. 28,214

ELECTROENCE PHALOGRAPH MONITORING APPARATUS 16- I V 20 i 3 L ,7 WWW. 1 L7 1 o 40 l 7 M X 1 ilk a M 60 i 1 l i 1 j I n /S ro IOOc/ f/QEGUENCYPIC-R6.

Oct. 29, 1974 o. E. MAYNARD Re. 28,214

ELECTROENCEPHALOGRAPH MONITORING APPARATUS Original Filed July 13. 19716 Sheets-Sheet 5 RELAXING OFF cARmAc ARREST assuscmmor CARDlAC ATTEMPTSMASSAGE Brew, {ABANDONED VOL TS DEATH PRONOUNCED 1974 o. E. MAYNARDELECTROEKCEPHALOGRAPH MONITORING APPARATUS 6 Sheets-Sheet f OriginalFiled July 13. 1971 United States Patent 28,214 ELECTROENCEPHALOGRAPHMONITORING APPARATUS Douglas Edward Maynard, Wentworth, Stoke PogesLane, England Original No. 3,699,947, dated Oct. 24, 1972, Ser. No.162,165, July 13, 1971, which is a continuation-in-part of abandonedapplication Ser. No. 771,192, Oct. 28, 1968. Application for reissueApr. 25, 1973, Ser. No. 354,461

Claims priority, application Great Britain, Oct. 31,

1967, 49,301/67 Int. Cl. A61b 5/04 US. Cl. 1282.1 B 16 Claims Matterenclosed in heavy brackets appears in the original patent but forms nopart of this reissue specification; matter printed in italics indicatesthe additions made by reissue.

ABSTRACT OF THE DISCLOSURE Monitoring equipment is provided formonitoring electrical signals in the human brain. Signals fromelectrodes attached to the scalp are passed to a band-pass filter wherecertain interference signals, such as those arising from muscle actionand rapid psycho-galvanic responses, are removed. The filter alsoapplies a weighting to equalize the effect of random brain activity overthe filter band, and to allow rhythmic activity to be more clearlyobserved. A logarithmic amplifier is connected to the filter output toprovide amplitude compression signals received from the brain. Theimpedance between the electrodes is monitored to give an indication ifthe electrodes become wholly or partially disconnected.

This application is a continuation-in-part of Ser. No. 771,192, filedOct. 28, 1968, now abandoned.

The present invention relates to apparatus for monitoring electricalsignals in the human brain and to apparatus for monitoring the impedanceof electrodes attached to the body and used particularly, but notexclusively, in the signal-monitoring apparatus.

In some circumstances, such as recovery from prolonged cardiac arrest,changes in the electroencephalogram (EEG) over a period of hours to afew days are important. Continuous EEG recording is not practical sinceit is excessively costly, so EEG sampling by multichannel recorders isused. It has been shown that the sampling techniques are inadequate inassessing a patients state and can sometimes give misleading results.

The present invention springs from the realization that the EEG showstwo main forms of activity: so-called random activity whose amplitudefalls with frequency, and rhythmic activity which is partly spread overthe same band as the random activity and partly takes the form ofsinusoidal signals of widely fluctuating amplitudes, those of greatestamplitude usually occurring in the range 8 to 12 cycles per second(c/s).

According to the present invention there is provided apparatus formonitoring electrical signals in the human brain, comprising a pair ofelectrodes, adapted to be fixed to the scalp, coupled to a filter of thetype hereinafter specified in which the lower predetermined frequency isabove 0.3 c/s and the upper predetermined frequency is below 30 c/s,respectively, and in which the frequency versus loss characteristicfalls with increase in frequency between the lower and upperpredetermined frequencies, so as to in operation substantially equalizethe amplitude of filter output signals due to random electrical activityof the brain, means for amplifying signals which pass through thefilter, and indicator means for indicating the amplitude of signals fromsaid amplifier means.

In this specification a filter of the type specified has a frequencyversus loss characteristic (that is the logarithm of frequency plottedagainst loss in decibels) in which loss decreases with increase infrequency at a generally constant rate from a low frequency to a highfrequency designated at the lower predetermined frequency and the upperpredetermined frequency, respectively. As frequency falls below thelower predetermined frequency, or rises above the upper predeterminedfrequency, loss increases at a rate greater than the said generallyconstant rate.

The importance of the frequency versus loss charac teristic of thefilter will now be appreciated; that is all frequency components of therandom activity falling within the passband of the filter are equallyrepresented, and the sinusoidal signals are prominent in the outputinstead of being swamped by the low-frequency random activity.

Preferably the apparatus includes a detector and a recorder and/ormeter.

By using the filter the trace provided by the recorder is converted froma form in which little information of a useful character can be obtainedto one in which changes in the activity of the brain can readily beobserved, especially relatively long term changes occurring in times offor example half an hour. Such changes are difficult or impossible toobserve on conventional EEG devices and often give considerable warning,of for example an hour, of cardiac arrest, an instant indication thatall is not well during surgical operations, and where resuscitation isbeing attempted an indication that success will be achieved many hoursbefore any other signs of return to consciousness are seen.

The output from this apparatus being in a simple form, can to a largeextent be interpreted by medical and nursing staff who have hadexperience of the apparatus. A far clearer indication of trends isobtained than from successive EEG recordings, and it is believed thatoperation could be carried out by relatively unskilled staff.

Since the apparatus is inexpensive, continuous monitoring is practicableand when used in conjunction with occasional EEG samples on amulti-channel recorder gives almost as much relevant information aswould be obtained by continuous EEG recording.

The frequency versus loss characteristic of the filter preferably fallsat a generally constant rate which provides a loss decrease of between 4and 12 db over the range 3 to 12 c/s.

Although in some circumstances it may be convenient if the filter hasthe frequency range mentioned above, the upper predetermined frequencyof the filter is preferably variable from 15 c/s to 5 c/s and the lowerpredetermined frequency is variable from 2 c/s to 12 c/s.

The electrodes are preferably coupled to the filter by way of a lownoise amplifier which may be a parametric amplifier, and the output fromthe filter is amplified by a logarithmic or semi-logarithmic amplifierin order to compress the amplitude of the output signal.

The impedance associated with electrodes attached to the body tends tobe subject to changes and, of course. the electrodes may becomecompletely disconnected. This is a weakness of the apparatus, althoughthe parametric amplifier can be designed to give only a 1 percent errorin gain when the electrode impedance has risen to about Kilohms. At suchimpedance however the apparatus becomes more sensitive to interference.

According to another aspect of the present invention therefore there isprovided apparatus for monitoring impedance between electrodescomprising a reactive component coupled between a pair of electrodes,each electrode being suitable for attachment to the human body, and thereactive component being coupled with a further impedance across theoutput of an oscillator, and phase comparison means for comparing thephase of two signals in the apparatus to provide an output signaldependent on the phase relationship of the said signals, the apparatusbeing such that if, when the electrodes are attached to a human body,the impedance between the body and the electrodes changes, then a changein the said phase relationship is indicated by the output signal of thephase comparison means.

The reactive component is preferably a capacitor and the furtherimpedance a resistor. The electrodes may, of course, be those of thesignal-monitoring apparatus, if the oscillator frequency is outside thepassband of the filter.

Certain embodiments of the invention will now be described by way ofexample with reference to the accompanying drawings in which:

FIG. 1 is a block diagram of apparatus according to one aspect of theinvention for monitoring electrical signals in the human brain;

FIG. 2 is a graph showing variation of electrical brain activity withfrequency;

FIG. 3 is a graph showing output signals from the filter 14 of FIG. 1;

FIG. 4 is a block diagram of apparatus according to another aspect ofthe invention for monitoring the impedance associated with electrodesattached to the body;

FIG. 5 is a circuit diagram of the apparatus according to FIG. 4 whichalso shows, partly by means of a block diagram, how the two monitors canbe combined;

FIG. 6 shows the loss versus frequency characteristic of the filter ofFIG. 1;

FIGS. 7 and 8 show the output from the recorder of FIG. 1 in dilferentcircumstances;

FIG. 9 is a part-circuit part-block diagram of the filter 14 and theamplifier 15 of FIG. 1; and

FIG. 10 is a loss versus frequency characteristic used in explaining thesetting up of the filter 14.

In FIG. l two silver-disc electrodes 10 and 11 are, in use, attached tothe scalp to pick up electrical signals in the brain. The electrodes arecoupled by way of fully screened cables whose characteristics do notvary with vibration i.e., the cables are non-microphonic to a low passfilter 12 which acts as a radio-frequency trap preventing high frequencyinterference reaching a low-noise parametric amplifier 13. A specialfilter 14 which has the weighted loss versus frequency characteristicshown in FIG. 6, is coupled to the output of the amplifier 13. Signalsfrom the filter are amplified first by a logarithmic amplifier 15 whichprovides amplitude compression, and then by an amplifier 16. A detector17 provides a signal for a meter 18 and a recorder 19, from the outputof the amplifier 16.

The Isleworth Electronics pre-amplifier A101 is suitable as theparametric amplifier 13, especially if specified as having selected lownoise input transistors, but many low noise differential amplifiers aresuitable such as the Burr Brown instrumentation amplifiers nos. 3161/and 3061/25.

The loss versus frequency characteristic of the filter 14 which is oneof the most important features of the invention will now be described,and then the construction of filters with this characteristic will bediscussed.

As can be seen from FIG. 6, to reduce the effect of interference oforiginating from muscle action potentials without rejecting significantrhythmic activity of cortical origin, the loss introduced by the filter14 rises abruptly above a frequency of about 12 c/s. Below a frequencybetween 2 and 3 c/s loss also increases to reduce the effects ofinterference originating from a number of sources such aspsycho-galvanic responses, sweating, and electrode movement. A highdegree of rejection is provided at frequency of the power supply (e.g.c/s in the UK. as shown in FIG. 6 or 60 0/5 in the U.S.A.) to reduce theproportion of amplified antiphase supply frequency components which canoccur at the input of the 4 parametric amplifier 13 when connected to asubject to whom other apparatus is attached.

Perhaps the most important part of the filter characteristic liesbetween about 3 c/s and 13 c/s and has been devised following therealization that the electrical activity of the brain can be representedin a simplified form as shown in FIG. 2 for the frequency rangeindicated where V is the voltage between electrodes fixed to the scalp.The random activity mentioned above is represented by the line 40, andthe rhythmic activity (that is alpha rhythm) by lines 41, 42 and 43.Since it is random, the random activity conveys no more information atany one frequency than it does at any other. To get a good estimate ofits level with a high degree of confidence (a large number of degrees offreedom), it is justifiable to weight the spectrum between about 3 c/sand 13 c/s as shown in FIG. 6 that is to have a decreasing loss withfrequency which rises at a rate of 7 db over a frequency range of 4to 1. This rate of decrease substantially equalizes the amplitude ofsignals, due to random electrical activity of the brain, at the filteroutput, and the rate can be obtained from the change in brain activitywith frequency of FIG. 2. Fluctuation of high frequency components nowhave the same degree of effect on the output as do those atlow-frequencies, as shown by the line 44 in FIG. 3. The rate at whichrandom activity varies depends to some extent on the person consideredand the 7 db rate mentioned above gives a practical filter for generaluse and represents an average rate. Individual variations are likely tobe within -3db to +5 db of the 7 db rate.

It is believed that the major component 43 of the rhythmic activityusually occurs at about 10 cps. If this were to have a small amplitudeand the spectrum were not weighted, fluctuation of this component wouldbe swamped by fluctuations of the low frequency random activity.However, by weighting the spectrum, fluctuations of this component aremore prominentas shown at 45 in FIG. 3. When this is added to the randomactivity, the width of the band written out by the recorder, where therecorder is a slow speed chart recorder, alters (because of the largeamplitude fluctuations of the rhythmic components), as does theamplitude distribution within the band.

Therefore the lower edge of the band is expected to indicate the lowestlevel usually reached by the random activity, assuming that the rhythmicactivity is not continuously present, while the upper edge of the bandindicates the highest level reached by the rhythmic and/or randomactivity. The width of the band, its height above the baseline, and theamplitude distribution within the band indicate the type of activity inthe signal. The rectified output of random activity would tend to have aRayleigh amplitude distribution. However, by using a logarithmicamplitude scale, this is converted to a more normal distribution, aswell as giving useful amplitude compression. For example, consideringthe output from the chart recorder, if a component at about 4 c/s with agiven amplitude variance were to be compared with a 10 c/s componentwith the same amplitude and variance, the width of the hand drawn by thechart recorder would be nearly the same in the two cases, but they wouldbe displaced by different amounts from the baseline, because of thefrequency loss characteristic of the filter. Conversely, if bothcomponents formed part of the random activity where the 4 c/s componentwould usually have a larger amplitude (see line 40 FIG. 2), thedisplacement for the two components and the width of the band would beapproximately the same.

The special characteristic of the filter 14 can be achieved in manyknown ways, and one such way will now be described in detail.

The major part of the characteristic above about 3 c/s is determined bya filter section 50 (see FIG. 9) of the type known as an unbalanced twinT" section. Such a filter section may be designed in the Way describedin Electronic Engineering for Apr. 1967 at page 219 by W. Farrer in anarticle entitled A Simple Active Filter with Independent Control OverPole and Zero Locations, to have at first a balanced characteristic asshown at 51 in FIG. with maximum rejection at a frequency of the powersupply. By successively adjusting potentiometers RS and R9 the section50 becomes progressively unbalanced and has the characteristics shown at52 and 53. By measuring the characteristic and adjusting thepotentiometers R5 and R9 the required characteristic above 3 c/s of FIG.6 is obtained. A resistor R1 and a capacitor C1 form a simple R-C lowpass filter which provides additional attenuation above 20 c/s.

Below 3 c/s the characteristic depends on a filter section 54 of thetype known as a Butterworth high pass filter. Such a filter may bedesigned in the way described in the Burr Brown Handbook of OperationalAmplifier Active Networks at page 82. This handbook is available fromthe Applications Engineering Section, Burr Brown Research Corporation,International Airport Industrial Park, Tuscon, Arizona.

The characteristic of FIG. 6 can be obtained with the component valuesgiven in Table 1 where the designations refer to FIG. 9.

TABLE 1 Components Value, K-Q R1 33 R2 and R3 120 R4 1 R5 and R9 Up to 1R6 and R7 22 Components Value or Type C1 0.22 t. C2, C3 and C6 0.1 f. C4and C5 0.5 ,uf. T1 and T4 2N 2484 T2 and T3 2 S 304 Instead of using afilter of the type shown in FIG. 9, the filter 14 may be designedaccording to the methods given by S. S. Haykin in his book Synthesis ofR-C Active Filter Networks" published by McGraw Hill, London 1969, seeparticularly Chapter 4.

Other ways are known in which filters with specific characteristics canbe designed and many of these ways are suitable for the design of thefilter 14.

The logarithmic amplifier may comprise an operational amplifier 55 withtwo silicon diodes 56 and 57, for example type 1844, connected in itsfeedback path. To provide a smooth gain characteristic for the amplifier55 at very low voltages where the diodes do not conduct a potentiometerR1!) of maximum resistance 50 K9 and a resistor R11 of resistance 100 K9are connected in parallel with the diodes. The amplifier 55 may be oneof the commercially obtainable operational amplifiers. The gain of theamplifier 15 applies amplitude compression which is then approximatelyproportional to the logarithm of the deviation of the input signal fromvirtual earth at the non-inverting input terminal of the operationalamplifier.

Examples of two outputs obtained from the apparatus of FIG. 1 are givenin FIGS. 7 and 8. The recording electrodes 10 and 11 were placed on thescalp of the subjects some two inches posterior to the vertex and some2% inches apart symmetrically disposed about the midline. This electrodeposition was chosen because this is the region in which the largest EEGactivity is usually recorded with a minimum of interference from muscleactivity, and because it records from both hemispheres simultaneously. Aground electrode, which can be omitted if electrical isolation of thesubject is required, was placed on the subjects forehead.

FIG. 7 shows the output from a normal subject performing mathematicalcalculations and drawing graphs. It can be seen that, over the recordedperiod, the variation of amplitude remained substantially constant, withoccasional larger deflections superimposed. In this case these largerdeflections coincided with bursts of alpha rhythms.

By comparison, FIG. 8 shows the output from a patient who had survived asevere cardiac arrest. The electroencephalogram recorded 15 minutesprior to the start of this record had little discernible activity apartfrom occasional large bursts of sinusoidal waves. It can be seen thatthe general level of activity steadily declined, to the point where asecond cardiac arrest occurred. There followed a brief period of highvoltage activity during unsuccessful cardiac massage.

In another case (not illustrated in the figures) a gradual rise in theband of activity from the baseline was the first indication by manyhours of the recovery of an unconscious patient suffering from anoverdose of barbiturates. Had this indication not been availableresuscitation might have seemed hopeless and abandoned.

The usefulness of the write-out can be extended. If the large spikesshown on the declining phase of FIG. 8 had occurred more frequently, theoutput would have been simply a thick band of activity. To avoid thispossibility, an electro-sensitive or heat sensitive wri1e-out system canbe used. In this case the write-out can be arranged to have the greatestintensity at those levels most frequently occurring where the pen of therecorder passes several times over the same point on the recordingmedium giving an amplitude distribution plot of the output by means ofvarying shades of intensity. This enables different types of EEGactivity having different amplitude distribution to be recognized eventhough they might have the same peak to peak range of variation.

For some Work it might prove desirable to have less low frequencyactivity present in the output. This can be arranged by providingvariable sections to the filter which gives the low frequency cut-offpoint at 2 c/s. By this means the operator can select the appropriatelow frequency cut-off point. A similar provision can be made for thehigher frequency cut-01f point at 15 c/s.

Apparatus for monitoring the impedance of apparatus associated with theelectrodes 10 and 11 is shown in FIG. 4.

An oscillator 21 with a frequency which is well above the frequencyrange of physiological activity of interest, and therefore outside thepassband of the filter 14. is applied across the electrodes 10 and 11through series resistors R51 and R52 which are large compared with theinput impedance of the amplifier 13. A small capacitor C19, theimpedance of which is negligibly high at the frequency of thephysiological activity but not at the frequency of the appliedoscillations is connected across the input terminals of the amplifier13.

The source of signals being monitored is represented by a sourceimpedance 25 and an oscillator 26. the impedance 25 being partlydependent on the conditions of contact between the skin and the discelectrodes.

When the source impedance 25 is small compared with the impedance of thecapacitor C19 at the applied frequency, there will be little phasedifference between the output signals of the oscillator 21 and thevoltage developed across the input terminals of the amplifier 13, sinceboth the capacitor C19 in parallel with the impedance 25 and thecomplete oscillator load (including the resistors R51 and R52, thecapacitor C19 and the impedance 25) are mainly resistive. However, ifthe impedance 25 increases so that it becomes comparable with or greaterthan the impedance of the capacitor C19, the impedance 25 and thecapacitor C19 taken together become more reactive and a phase lag up toa maximum of 90 degrees is produced at the amplifier input.

To prevent the monitoring voltage applied across the capacitor C19 fromdisappearing if the electrodes are short circuited, small resistors R53and RS4 of negligible impedance compared to the input impedance of theamplifier 13 are connected in series with the electrode leads.

At the output of the amplifier 13 the signal from the oscillator 21 isseparated from the signals from the source 26 by a frequency selectiveamplifier 30 and formed into a square-wave by a clipping amplifier 31.To compare the phase of the square-wave signal with that of the outputsignals of the oscillator 21, the square-wave signal is applied to anAND gate 32, together with signals from the oscillator 21 which havebeen formed into a squarewave by a clipping amplifier 33.

When signals across the capacitor C19 and at the output of theoscillator 21 are in phase, signals arriving at the AND gate 32 by thetwo paths are out of phase,

due to an inversion arranged in one path. Thus the AND gate is not openfor in phase oscillator and capacitor signals, but opens, for increasingperiods, as the phase difference increases. Consequently the magnitudeof the signal from a smoothing network 34 depends on the phasedifference and hence on the source impedance 25.

A recorder 35 gives a continuous record of the condition of theelectrodes and a trigger circuit 36 operates an alarm 37 when the sourceimpedance rises to an unacceptable level. The trigger circuit 36 may beof any suitable known voltage sensitive type.

A further trigger circuit may be added so that if the impedance fallsbelow a given level, indicating an input short circuit, another alarmcircuit may be operated. This alarm circuit may be of such a nature thatthe chart recorder pen is deflected from the recording area, preventingthe recording of inaccurate information.

A further features of the impedance monitor is that it can be arrangedto give an indication if the signal from the oscillator 21 is preventedfrom passing from the amplifier 13 to the amplifiers 30 and 31 byblocking of the amplifier 13 or by circuit failure. This is accompaniedby adjusting the bias of clipping amplifier 31 so that, in the absenceof an input from amplifier 30 the input to the AND gate 32 switches tothe on" state. Under normal circuit conditions, with a very low sourceresistance 25, the two inputs to the gate 32 are 180 out of phase andthe gate does not open. With a very large source resistance, the twoinputs to the gate are 90 out of phase and the gate opens for onequarter of a cycle. With no input to the amplifier 31 the gate isswitched solely to the clipping amplifier 33 an input from the amplifier31 being present permanently, and is open for alternate half cycles.

Therefore it can be seen that, if the output voltage from the smoothingnetwork 34 is zero for a very small source resistance 25 and is V for avery large source resistance then, if the input to the clippingamplifier 31 is removed, the output becomes 2V. Other outputconfigurations can be achieved. For example, by including a 90' phaseshift in amplifier 30 the output from the smoothing network 34 can bezero for large source resistance, V for zero source resistance, and 2Vfor no input to amplifier 31.

While suitable circuits for the impedance monitor are well known, acircuit diagram for the above described impedance monitoring equipmentincluding a 90' phase shifter in the amplifier 30 is shown in FIG. 5.

A resistor R12 and a capacitor C7 in the amplifier 30 together with theinput impedance of a transistor T in parallel with the resistors R13 andR form a high pass filter which attenuates frequencies having aphysiological origin but which permits the impedance monitoring signalto pass with less attenuation. A capacitor C11 and a resistor R with theinput impedance of a 8 biasing resistor R28 have the same effect as do acapacitor C9, a resistor R17, the input impedance of a transistor T6 inparallel with a resistor R20. Thus signals of physiological origin areprevented from reaching the clipping amplifier 31.

The resistors R53 and R54 and the capacitor C19 are used as part of theRF. trap 12. It is convenient to use a multi-vibrator as the oscillator21, although a squarewave source produces more complicated phase changesacross the capacitor C19 than a sine wave. Performance is not muchdegraded but some phase adjustment may be desirable at the output of theamplifier 13. Since the multivibrator provides a square-wave theclipping amplifier 33 is not required.

Two complementary outputs are taken from the multivibrator to drive twosubordinate AND gates made up of diodes D1 and D2, and D3 and D4,respectively. The clipping amplifier 31 also acts as a phase splitterwith the collectors of transistors T10 and T11 driven in antiphase. Thusthe two subordinate AND gates operate in antiphase and charge issupplied to capacitor 13 twice as frequently as would otherwise occurand the amplitude of the output signal applied to a buffer amplifier 38,provided to drive the recorder and the trigger circuit is doubled. The90 phase shift in the amplifier 30 is obtained at the junction of thecapacitor C8 and the resistor R17 which are driven from antiphaseoutputs at the collector and emitter of the transistor T5.

Where it is required to indicate loss of the impedance monitoring signalgenerated by the multivibrator 21, the diodes D3, D4 and D6 are omitted,thus one gate only is provided. Also a resistor (not shown) is addedbetween the base of the transistor T11 and the terminal to which the8.5V bias is applied. This resistor is individually selected to providea suitable bias for the AND gate when the monitoring signal is absent.The loss of the input signal then causes the gate to be open for onehalf of each cycle, zero impedance at the electrodes 10 and 11 opens thegate for a quarter of each cycle and with high impedance the gate isclosed during the whole cycle. With such an arrangement the alarmcircuit which detects a short circuit input may also be used to detectloss of the monitoring signal.

The circuit of FIG. 5 may be constructed using the components given inTABLE II, where the designations used are those shown in FIG. 5.

TABLE II Components Value R 14, R16, R38 "K9" 1 R18, R19, R28 KQ 10 R32,R42, R "KS2" 10 R20, R22, R33, R34 Kn 22 R39, R47 Ko 22 R12, R15 KQ 47R21, R24, R25 KSl 2.2 R30, R31 "Kn" 2.7 R53, R54 "KS2..- 2.2 R41, R46,R49, R "K0" 4.7 R13 Kn 120 R17 Kn 1.5 R23 MS2 1 R26 KQ 6,8 R27 K0-.. 3.3R29 -12-- 680 R37 "KO" 330 R40, R48 "KS2" 56 R43, R44 Ko 82 R51, R52 MnComponents Value or Type C8, C11, C13 0.1 M. C7, C14, C15 0.01 pf. C90.047 pf. C12, c10 1,.f. C16, C17 0.005 ,uf. C19 0.001 pf. C18 0.002 t.T5, T6, T7, T13 2 N 3707 T10, T11, T9, T8 2 N 3703 T12 2 S 304 T14, T152 N 1304 D1, D2, D3 A 91 D4, D5, D6 0 A91 D7, D8 0 A 91 The arrangementof FIGS. 4 and is inherently insentive to changes of gain of theamplifier 13 and since the monitoring voltages are alternating theelectrodes are not polarized.

The impedance monitoring apparatus has been specifically described foruse with the signal-monitoring apparatus but it could of course be usedwherever the impedance associated with electrodes is important.

It is expected that in addition to monitoring of EEG following cardiacarrests the signal-monitoring apparatus will find a useful applicationin monitoring EEG under long term anaesthesia, with particular emphasison open heart surgery where the maintenance of a sufficient blood supplyto the brain is of critical importance. Apart from warning of imminentcortical extinction, the clear representative of trends should prove tobe more convenient than retrospective visual comparisons ofconventionally recorded EEG.

A further intended application of the apparatus for monitoring EEG is instudying the effect of drugs and the monitoring of long term sleepchanges under various conditions.

I claim:

1. Apparatus for monitoring electric signals in the human brain,comprising:

a pair of electrodes adapted to be fixed to the scalp,

filter means coupled to said electrodes, having a frequency versus losscharacteristic in which loss decreases with increasing frequency so asto, in operation, substantially equalize the ampliture of the filteroutput signals due to random electrical activity of the brain, the saiddecrease in loss with increase in frequency being from a low frequencydesignated the lower predetermined frequency, to a high frequencydesignated the upper predetermined frequency, the upper and lowerpredetermined frequencies being below 30 c/s and above 0.3 c/s,respectively, and loss below the lower predetermined frequency, andabove the upper predetermined frequency, being greater than the loss atthe lower and upper predetermined frequencies, respectively,

amplifying means for amplifying signals which pass through said filtermeans, and indicator means for indicating the magnitude of signals fromsaid amplifying means. 2. Apparatus according to claim 1, wherein thesaid filter means has a characteristic, plotted as the logarithm offrequency against loss in decibels, such that said decrease in loss withincrease in frequency is at a generally constant rate providing a lossdecrease of between 4 and 12 decibels over the range 3 to 12 c/s.

3. Apparatus according to claim 1, wherein said lower predeterminedfrequency of said filter means is substantially 2 c/ s.

4. Apparatus according to claim 1, wherein said lower predeterminedfrequency of said filter means can be varied from 2 to 12 c/s.

5. Apparatus according to claim 1, wherein said upper predeterminedfrequency of said filter means is substantially 15 0/5.

6. Apparatus according to claim 1, wherein said upper predeterminedfrequency of said filter means can be varied from 8 to 20 c/s.

7. Apparatus according to claim 1, comprising low-noise amplifier meanscoupled between said electrodes and said filter means.

8. Apparatus according to claim 1, wherein said amplifying meansincludes a logarithmic amplifier.

9. Apparatus according to claim 1, wherein said indicating meansincludes a detector circuit coupled to said amplifying means and arecorder coupled to said detector means having a pen to mark a recordingmedium, said recorder being adapted to provide a visible record whoseintensity at a point depends on the number of times said pen traversessaid point.

10. Apparatus according to claim 1, including monitoring means formonitoring the impedance between said electrodes, comprising reactivemeans coupled between said electrodes,

oscillator means coupled across said reactive means and a furtherimpedance,

phase-comparison means for determining the phase relationship of twosignals in the monitoring means to provide an output signal, dependenton the phase relationship of the said signals, giving an indication ofchange in impedance between said electrodes, and

indicating means, for indicating said change of phase relationship,coupled to the output of said phase comparison means.

11. Apparatus according to claim 10 wherein said reactive means is acapacitor and the further impedance is a resistor.

12. Apparatus according to claim 11 wherein the capacitor is coupled inseries with the resistor across the output of the oscillator and thephase comparision means compares the oscillator output voltage with thevoltage across the capacitor.

13. Apparatus according to claim 12 wherein said phase comparison meansincludes first and second means for forming square-waves from saidsignals from said oscillator and across said reactive means,respectively, and

AND gate means adopted to receive said square-wave signal as inputs, thetime during which said AND gate means is open providing said indicationof said phase relationship.

14. Apparatus according to claim 13, wherein said AND gate isconstructed to become permanently in one state if there is no path forsignals from said oscillator by way of said reactive means.

15. Apparatus according to claim 10, wherein the frequency of saidoscillator is above the upper predetermined frequency of said filtermeans,

said oscillator is coupled to said electrodes between said electrodesand said filter;

said reactive means is coupled to the input of said filter, andfrequency-selective means is provided which is adapted to reject allfrequencies present at the input to said filter except that of saidoscillator, the input of said frequency selective means being coupled inparallel with the input of said filter, and the output of said frequencyselective means is coupled to said phase comparison means.

16. Apparatus for monitoring electrical signals in the human brain,comprising:

a pair of electrodes adapted to be fixed to the scalp,

filter means coupled to said electrodes, having a frequency versus losscharacteristic in which loss decreases with increasing frequency at agenerally constant rate providing a loss decrease of between 4 11 12 and12 decibels over the range 3 to 12 c/s, and 3,123,768 3/1964 Burch etal. 1282.1 X loss increases with decrease in frequency below, 3 1 5 9255 19 5 Grass 3 and with increase in frequency above, the said range 3195 533 7/1965 Fischsr 128 2'1 at a greater rate than the said rate,amplifying means for amplifying signals which pass through said filter 53212496 10/1965 Preston 1282'1 X means, and indicator means forindicating the magni- 7, 3 0/1 67 PaCcIa 1282.1 tude of signals fromsaid amplifying means. 3,452,743 7/1967 Rieke 128-21 References CitedKYLE L. HOWELL, Primary Examiner The following references, cited by theExaminer, are 10 U S Cl X R of record in the patented file of thispatent or the original patent 128-2.1 Z

UNITED STATES PATENTS 2,490.033 10/1946 Garceau 1282.1

2,657,683 11/1953 Koller 12s 2.1 15

